When transferring power using inductive resonance, effectively creating a loosely coupled transformer, why is it important that the frequency of the input needs to be the same as the resonating frequency for voltage to be induced in the secondary?

In the attached image a function generator of 50Hz is used to feed a resonance circuit of around 90kHz as a demonstration of what will not work.

Thanks!

 

In order to develop much voltage across a resonant tuned circuit the applied magnetic field must be of the same frequency. And the lower the resistance of the coil the more efficient it becomes, but also the frequency matching becomes even more critical. THAT is the reason that matching frequencies is important. Why are you experimenting with this if you have not even learned that very fundamental detail? First, understand the physics and the mathematics of the coupling and then the answer will be very obvious.
In short: Resonant inductive coupling does not exist without resonance. And that resonance must be at the frequency that the circuit is resonant at.